1. Field of the Invention
The present invention is directed to brushless DC motors, and more particularly to a method and apparatus for ripple suppression in brushless DC motors having limited drive/amplifier bandwidth when operated at high velocity.
2. Description of the Related Art
Brushless DC (BLDC) motors, or electronically commutated motors (ECMs, EC motors) are electric motors that are powered by direct-current (DC) electricity via electronic commutation systems. BLDC motors exhibit linear current-to-torque and frequency-to-speed relationships and are commonly used as servo drives for precision motion control in numerous applications ranging from silicon wafer manufacturing, medical, robotics and automation industries to military applications.
BLDC motors comprise a rotor having a plurality of permanent magnets, a stator with electromagnetic coil armature windings, and a commutator for continually switching the phase of the current in the armature windings to induce motion in the rotor. More particularly, an electronic controller causes the commutator to apply excitation current to the armature windings in a specific order in order to rotate the magnetic field generated by the windings thereby causing the rotor magnets to be pulled into alignment with the moving magnetic fields and thereby drive rotation of the rotor.
Prior art relating generally to the construction of BLDC motors and controllers therefor, includes: U.S. Pat. Nos. 6,144,132; 7,715,698; 7,852,025; 7,906,930; US 2010/0109458; US 2010/01487101; US 2010/0176756; US 2010/0181947; US 2010/0270957; US 2010/0314962; US 2011/0031916; US 2011/0074325; US 2011/0133679. The following patent documents are directed to methods of controlling speed (velocity) of BLDC motors: U.S. Pat. Nos. 4,720,663; 4,855,652; 5,563,980; 5,757,152; 6,049,187; 6,313,601; 6,822,419; 6,828,748; 6,922,038; 7,375,488; 7,994,744; US 2005/0035732; US 2009/0128078; US 2010/0001670; US 2010/0134055; US 2010/0171453.
In order to control the speed of rotor rotation, the electronic controller requires information relating to the rotor's orientation/position (relative to the armature windings). The following patent documents are directed to methods of commutating and controlling BLDC motors relying on feedback of the sensed rotor position: US 2010/0117572; US 2010/0134059; US 2010/0237818; US 2010/0264862; US 2010/0270960; US 2011/0043146; US 2011/0176229.
Some electronic controllers use Hall Effect sensors or rotary encoders to directly measure the rotor position. Other controllers measure the back EMF in the windings to infer the rotor position. The following patent documents disclose methods of commutating and controlling BLDC motors relying on feedback of sensed back electromotive force (back EMF): U.S. Pat. Nos. 5,057,753; 5,231,338; US 2011/0074327; US 2011/0115423; US 2011/0121770; US 2011/0156622; US 2011/0210688. Additional background patent literature relevant to commutating BLDC motors using feedback includes: U.S. Pat. Nos. 7,893,638; 7,969,108; US 2010/0052584; US 2010/0090633; US 2010/0237813; US 2010/0237814; US 2010/0315027; US 201110006712; US 201110025237; US 201110043144; US 201110084639; US 201110148336; US 201110202941; US 201110205662.
Typically, the excitation current generated by the commutator has insufficient power to be fed directly to the coils, and must be amplified to an appropriate power level by a driver/amplifier. The conventional driver/amplifier of a BLDC motor produces sinusoidal armature current waveforms to the armature windings for smooth motor operation. However, in practice, the actual magneto-motive force generated by a non-ideal motor is not perfectly sinusoidally distributed, and can therefore result in ‘torque ripple’. It is known that suppressing torque ripple in the motor drive of a servo system can significantly improve system performance by reducing speed fluctuations (see Park, S. J., Park, H. W., Lee, M. H. and Harashima, F.: A new approach for minimum-torque-ripple maximum-efficiency control of BLDC motor, IEEE Trans. on Industrial Electronics 47(1), 109-114 (2000); and Aghili, F., Buehler, M. and Hollerbach, J. M.: Experimental characterization and quadratic programming-based control of brushless-motors, IEEE Trans. on Control Systems Technology 11(1),139-146 (2003) [hereinafter Aghili et al., 2003].
One known solution to reducing torque ripple in commercial high-performance electric motors is to increase the number of motor poles. However, such motors tend to be expensive and bulky due to the construction and assembly of multiple coil windings.
Other control approaches for accurate torque control in electric motors, and their underlying models, are set forth in the patent literature. For example, the following patent documents are directed to various methods and systems for stabilizing or reducing torque ripple in synchronous electric motors, particularly BLDC motors: U.S. Pat. Nos. 4,511,827; 4,525,657; 4,546,294; 4,658,190; 4,912,379; 5,191,269; 5,569,989; 5,625,264; 5,672,944; 6,437,526; 6,737,771; 6,859,001; 7,166,984; 7,859,209; 7,952,308; US 201110175556.
Additional prior art approaches for providing accurate torque production in electric motors are set forth in the non-patent literature.
For example, Le-Huy, H., Perret, R. and Feuillet, R.: Minimization of torque ripple in brushless dc motor drives, IEEE Trans. Industry Applications 22(4), 748-755 (1986), and Favre, E., Cardoletti, L. and Jufer, M.: 1993, Permanent-magnet synchronous motors: A comprehensive approach to cogging torque suppression, IEEE Trans. Industry Applications 29(6), 1141-1149, describe a method of reducing the torque-ripple harmonics for brushless motors by using several current waveforms.
Ha and Kang: Explicit characterization of all feedback linearizing controllers for a general type of brushless dc motor, IEEE Trans. Automatic Control 39(3), 673-6771994 (1994) characterize, in an explicit form, the class of feedback controllers that produce ripple-free torque in brushless motors.
Newman, W. S. and Patel, J. J.: Experiments in torque control of the AdeptOne robot, Sacramento, Calif., pp. 1867-1872 (1991) discuss the use of a 2-D lookup table and a multivariate function to determine the phase currents of a variable-reluctance motor with respect to position and torque set points.
Optimal torque control schemes for reducing torque ripples and minimizing copper losses in BLDC motors have been proposed (see Hung, Y and Ding, Z.: Design of currents to reduce torque ripple in brushless permanent magnet motors, IEEE Proc. Pt. B 140(4) (1993) [hereinafter Hung-Ding 1993]; Aghili, F., Buehler, M. and Hollerbach, J. M.: Optimal commutation laws in the frequency domain for PM synchronous direct-drive motors, IEEE Transactions on Power Electronics 15(6), 1056-1064 (2000) [hereinafter Aghili et al., 2000]; Park, S. J., Park, H. W., Lee, M. H. and Harashima, F.: A new approach for minimum-torque-ripple maximum-efficiency control of BLDC motor, IEEE Trans. on Industrial Electronics 47(1), 109-114 (2000); as well as Aghili et al., 2003, above.
Wang, J., Liu, H., Zhu, Y., Cui, B. and Duan, H.: 2006, A new minimum torque-ripple and sensorless control scheme of bldc motors based on rbf networks, IEEE Int. Conf. on Power Electronics and Motion Control, Shanghai, China, pp. 1-4 (2006), proposes a method for minimizing the torque ripples generated by non-ideal current waveforms in a BLDC motor having no position sensors, by adjusting actual phase currents.
Similarly, the electrical rotor position can be estimated using winding inductance, and the stationary reference frame stator flux linkages and currents can be used for a sensorless torque control method using d-axis current, as set forth in Ozturk, S. and Toliyat, H. A.: Sensorless direct torque and indirect flux control of brushless dc motor with non-sinusoidal back-EMF, IEEE Annual Conf. on Industrial Electronics IECON, Orlando, Fla., pp. 1373-1378 (2008).
Lu, H., Zhang, L. and Qu, W., A new torque control method for torque ripple minimization of BLDC motors with un-ideal back EMF, IEEE Trans. on Power Electronics 23(2), 950-958 (2008), sets forth a torque control method to attenuate torque ripple of BLDC motors with non-ideal back electromotive force (EMF) waveforms, wherein the influence of finite dc bus supply voltage is considered in the commutation period.
A low cost BLDC drive system is set forth in Feipeng, X., Tiecai, L. and Pinghua, T.: A low cost drive strategy for BLDC motor with low torque ripples, IEEE Int. Conf. on Industrial Electronics and Applications, Singapore, pp. 2499-2502 (2008), wherein only a current sensor and proportional-integral-derivative controller (PID controller) are used to minimize the pulsating torque.
In the prior art set forth above, it is assumed that the phase currents can be controlled accurately and instantaneously and that they may therefore be treated as control inputs, such that the waveforms of the motor phase currents may be adequately pre-shaped so that the generated torque is equal to the requested torque. However, at high rotor velocity the commutator generates high frequency control signals that the finite bandwidth motor dynamics of the driver/amplifier may not be able to respond sufficiently quickly. Thus, complete compensation for the position nonlinearity of the motor torque cannot be achieved in the presence of amplifier dynamics, with the result that pulsation torque therefore appears at high motor velocities. As discussed above, torque ripple can significantly deteriorate the performance of the servo control system and even lead to instability if the ripple frequency is close to the modal frequency of the closed-loop system.
In Aghili et al., 2000, above, an optimal commutation scheme is set forth based on Fourier coefficients in BLDC motors. This was followed by Aghili, F.: Adaptive reshaping of excitation currents for accurate torque control of brushless motors, IEEE Trans. on Control System Technologies 16(2), 356-364 (2008), which developed a self-tuning adaptive version of the commutation law that estimates the Fourier coefficients of the waveform associated with the motor's electromotive force based on measurements of motor phase voltage and angle.
Other prior art has addressed the application of Fourier analysis to BDLC motor control. For example, U.S. Pat. No. 6,380,658 discloses a method and apparatus for torque ripple reduction in a sinusoidally excited brushless permanent magnet motor for automotive applications (for electric power steering, as an alternative to hydraulic power steering). In essence, the physical components of the BLDC motor are designed to a predetermined geometry for reducing torque ripple when the motor is sinusoidally excited (i.e. driven by a sinusoidal current supplied to its armature coils). More specifically, the method of the '658 patent applies an elementary Fourier analysis to determine one specific dimension of the rotor of a given shape which minimizes the fifth harmonic component of the magnetic flux in the air gap between the stator and the rotor, when the motor is driven by a sinusoidal current. The fifth harmonic is identified as the lowest harmonic having adverse influence on torque ripple and therefore the one that should be eliminated to the extent possible.
U.S. Pat. No. 7,629,764 discloses a method of controlling a high-speed servomotor, such as a BLDC motor, operating under control of a PWM (pulse-width modulation) controller, to attain optimal performance and stability margins across an operational range encompassing the entire torque versus speed curve of the motor. According to the '764 patent, the torque versus speed curve for the motor is divided into operating regions, and control parameters are calculated for each region. Fourier analysis (transforms) of various feedback signals received from the operating motor (e.g., the electric current flowing through the motor and its actual speed) is carried out in real time to produce fundamental and harmonic components of the feedback signals, which components are then used to produce an output voltage command for controlling the motor.
Additional patent literature is directed to methods and systems for controlling and/or operating electric motors using Fourier transforms of signals, including: FR 2,825,203; JP 2001238,484; JP 2007143,237; U.S. Pat. Nos. 4,7440,41; 5,280,222; 5,455,498; 5,844,388. The foregoing patent references disclose various methods for controlling or commutating electric motors, in particular BLDC motors, using the results of Fourier transforms of various periodic signals, feedback or otherwise.
Additional non-patent literature relevant to this disclosure includes:                Murai, Y, Kawase, Y, Ohashi, K., Nagatake, K. and Okuyama, K.: 1989, Torque ripple improvement for brushless dc miniature motors, Industry Applications, IEEE Transactions on 25(3), 441-450;        Delecluse, C. and Grenier, D.: 1998, A measurement method of the exact variations of the self and mutual inductances of a buried permanent magnet synchronous motor and its application to the reduction of torque ripples, 5th International Workshop on Advanced Motion Control, Coimbra, pp. 191-197;        Wallace, R. S. and Taylor, D. G.: 1991, Low-torque-ripple switched reluctance motors for direct-drive robotics, IEEE Trans. Robotics & Automation 7(6), 733-742;        Filicori, E, Bianco, C. G. 1. and Tonielli, A.: 1993, Modeling and control strategies for a variable reluctance direct-drive motor, IEEE Trans. Industrial Electronics 40(1), 105115;        Matsui, N., Makino, T. and Satoh, H.: 1993, Autocompensation of torque ripple of direct drive motor by torque observer, IEEE Trans. on Industry Application 29(1), 187-194;        Taylor, D. G.: 1994, Nonlinear control of electric machines: An overview, IEEE Control Systems Magazine 14(6), 41-51;        Kang, J.-K. and Sui, S.-K.: 1999, New direct torque control of induction motor for minimum torque ripple and constant switching frequency, IEEE Trans. on Industry Applications 35(5), 1076-1082.;        French, G. and Acamley, P.: 1996, Direct torque control of permanent magnet drives, IEEE Trans. on Industry Applications 32(5), 1080-1088;        Kang, J.-K. and Sui, S.-K.: 1999, New direct torque control of induction motor for minimum torque ripple and constant switching frequency, IEEE Trans. on Industry Applications 35(5), 1076-1082;        Xu, Z. and Rahman, M. F.: 2004, A variable structure torque and flux controller for a DTC IPM synchronous motor drive, IEEE 35th Annual Power Electronics Specialists Conference, PESC04., pp. 445-450, Vol. 1).        